The production of organic compounds using synthesis gas, which is a mixture of carbon monoxide and hydrogen, or from carbon monoxide as one of the reactants has been known for a significant period of time. It is well known that one can produce methanol directly from synthesis gas and that methanol can be further reacted by hydroformylation, homologation and carbonylation reactions to produce acetaldehyde, ethanol and acetic acid or its methyl ester, respectively. It is also known that alcohols, esters, ethers, and other organic compounds can be reacted with synthesis gas or carbon monoxide to produce oxygenated organic compounds. The difficulties. however, have resided in the ability to carry out any one of these chosen reactions to produce the desired compound at acceptable efficiency, conversion rate and selectivity.
In almost all instances the reaction is generally catalyzed using a Group VIII transition metal compound as the catalyst and a halogen as the promoter. It is known that many other metal compounds and promoters can be used. In addition, the prior art has disposed the use of secondary activators or ligands in conjunction with the metal catalysts and promoters. These secondary activators can be other metallic salts or compounds, amines, phosphorus compounds, as well as a multitude of other compounds that have been disclosed in the published literature. Thus, a typical catalyst system contains the metal atom catalyst, promoter and, optionally, ligands, solvents and secondary activators.
The prior art has taught that a wide variety of Group VIII metals and a halide promoter will catalyze the carbonylation of methanol to methyl acetate. For most Group Vlll metals the catalyst system requires extremely high operating pressures. Several patents describing such processes are discussed hereafter.
U.S. Pat. No. 4,212,989, issued to Isshiki et al.. on July 15 1980, describes a process for producing carboxylic acids or their esters by reacting an alcohol or an ether with carbon monoxide using a Group VIII metal catalyst and an iodine promoter. The reference contains no disclosure or suggestion of the production of acetate esters employing a specific rhodium complex under mild reaction conditions.
Japanese patent publication 55-160743. filed by Mitsubushi and published on Dec. 13, 1980 discloses reaction of methanol with a mixture of syn gas and an aluminum catalyst to produce methyl acetate and methyl formate.
The catalyst is charged as aluminum acetylacetonate or the like. Halide promoters are not required. The reaction is typically carried out at pressures from 1700 to 3000 psi with the ratio of hydrogen to carbon monoxide ranging from 1:1 to 0.1:1.
U.S. Pat. No. 4,166,189 issued Aug. 28, 1979 discloses a process for preparing branched methyl esters by the reaction of methanol with carbon monoxide in the presence of a zinc iodide catalyst. Metal acetate was observed only in trace amounts, with the principal products being methyl pivalate and methyl-2,2,3,3,-tetramethylbutyrate. The carbon monoxide partial pressure in the reaction mixture is said to be preferably greater than 1000 psia and the reaction temperature is most preferably from 200 to 280.degree. C. There was no co-feed of another alcohol with the methanol.
Japanese Patent Publication No. 56-104839, filed by Kurrary and published on Aug. 20, 1981 discloses that methyl acetate may be obtained by reacting methanol and carbon monoxide employing a heterogenous catalyst, which includes nickel and/or cobalt-rare earth oxide-and a metal from the platinum group. The rate of methyl acetate formation is very low, usually much less than 1.0 mole per hour and there is no disclosure of co-feeding an alcohol with the methanol.
U.S. Pat. No. 3,856,856, issued Dec. 24, 1974 discloses that methyl acetate may be formed by reacting methanol with carbon monoxide in the presence of a catalyst of the formula Co--I--Pt--ER.sub.3, wherein E is N, P, As and R is an organic moiety. Operating temperatures are generally preferred to be from 100-200.degree. C. and preferred operating pressures are in the order of 1000 to 4000 psig. The rate of methyl acetate production is less than 1 mole per liter per hour and there is no disclosure of employing mixtures of different alcohols for the alcohol feed.
Japanese Patent Publication 51-80813, filed by Mitsui Petro Chemical was published on July 15, 1975 and relates to forming methyl acetate by reacting methanol with carbon monoxide employing a Re-I catalyst. It was reported that at 200.degree. C. and 1100 psi the methyl acetate production rate was 1.0 mole per liter per hour and the selectivity was above 40%.
U.S. Pat. No. 4,134,912. issued Jan. 16, 1979 teaches production of methyl acetate by the reaction of methanol with carbon monoxide in the presence of a Ni--Sn--CH.sub.3 I catalyst. Although it is stated that a wide range of temperatures is suitable it is disclosed that best results are employed when the reaction temperature is from 125-225.degree. C. The process is said to be carried out at partial pressures from 1 to 10,000 psi, but in the actual results reported, the reaction pressure was maintained at from 350-500 psig.
U.S. Pat. No. 4,250,329 issued Feb. 10, 1981 discloses preparation of esters by the reaction of an alcohol with carbon monoxide in the presence of a halide promoter and a catalyst of the general formula: BxMN[M'(CO).sub.a (L).sub.b ].sub.2, wherein N is a Group II A metal; Bx is a Lewis base, such as pyridine or THF; M' is a Group VI, VII or VIII metal; L is a derivative of cyclopentadiene and/or BR.sub.3, where B is N, P, As, or Sb and R is an organic alkyl. That catalyst is different from the novel rhodium catalyst employed herein.
It is also said in U.S. Pat. No. 4,250,329 that where the carbon monoxide pressure ranges from 1000 to 4000 psig. ester formation is favored. Such reaction conditions especially reaction pressure, are rather harsh. It is further said that reaction temperatures from 100-200.degree. C. favor ester formation. It is also noted that the process is applicable to production of unsymmetrical esters, such as by mixing methanol and isopropanol to yield methyl acetate, isopropyl acetate and small amounts of methyl propionate and methyl isopropionate. Such reaction conditions, especially reaction pressure are rather harsh.
U.S. Pat. No. 3,769,329 issued Oct. 30, 1973 relates to forming acetic acid by carbonylating methanol with a Rh-I
In several examples, notably Examples 27-29, a process for producing methyl acetate is illustrated. In those examples the reaction temperature was 175.degree. C. and the reaction pressure, 500 psig. At such conditions well beyond those of the instant invention the methyl acetate rate was about 3 % moles per liter per bour and selectivity was 90%.
The above-noted patents demonstrate that a wide variety of Group VIII metals and a halide promoter will catalyze the carbonylation of methanol to methyl acetate. In general, however such catalysts require high operating pressures, usually, on the order of from 3000 to 5000 psi. A rhodium catalyst functions at somewhat lower operating pressures on the order of 500 psi but only when reaction temperatures are maintained on the order of 180.degree. C.
Typical prior art processes employing various Group VIll catalysts to produce acetate esters require rather harsh reaction conditions of temperature and pressure to obtain satisfactory yields of products. Such r.RTM.action conditions require use of expensive reactors engender excessive energy cost, often lead to undesired by-products and cause excessive corrosion problems.